the genetically inherited condition
in which there is a marked deficiency of pigmentation in skin, hair, and eyes. An
individual with these traits is an "albino." Since the gene for albinism
is recessive, it only shows up in the phenotype of
homozygous recessive people. This is a
pleiotropic
trait.

alternate forms or varieties of a
gene. The alleles for a trait occupy the same locus or position on
homologous chromosomes and thus govern
the same trait. However, because they are different, their action may
result in different expressions of that trait.

small molecules that
are the components of proteins. There are 20 different kinds of amino acids in living
things. Proteins are composed of different combinations of amino acids assembled in
chain-like molecules. Amino acids are primarily composed of carbon, oxygen,
hydrogen, and nitrogen.

a rare genetically inherited form
of mental retardation due to the deletion or inactivation of specific genes
on chromosome 15. The inheritance of this syndrome is subject to
genome imprinting.
Children with Angleman syndrome typically also have small heads,
experience seizures, have pronounced speech impairment, are hyperactive,
and have balance disorders.

an incorrect 19th century theory
about the inheritance of characteristics. It proposed that inherited
traits blend from generation to generation. Through his plant
cross-breeding experiments, Gregor Mendel proved that this was wrong

an individual who is heterozygous for a trait that only shows up in the
phenotype of those who are homozygous
recessive. Carriers often do not show any signs of the trait but can pass it on to
their offspring. This is the case with hemophilia.

thread-like,
gene-carrying bodies in the nucleus of a cell. Chromosomes are
composed primarily of DNA and protein.
They are visible only under magnification during certain stages of cell
division. Humans have 46 chromosomes in each somatic
cell and 23 in each sex cell.

the situation in which two
different alleles for a trait are expressed unblended in the
phenotype of heterozygous
individuals. Neither allele is dominant or recessive, so that both
appear in the phenotype or influence it. Type AB blood is an example. Such traits are said to be codominant.

a genetically inherited disease
in children that results in chronic fluid development in the lungs, making
breathing difficult. This disease also prevents normal absorption of
fats and other nutrients from food. Cystic fibrosis occurs as a
result of inheriting a recessive allele for it from both parents.
This is ultimately a fatal disease, but with modern medical care, about 2/3
of the people with it survive into early adulthood. About 30,000
people have cystic fibrosis in the U.S. today.

an inherited metabolic disorder in
which there are abnormally high blood sugar levels. In advanced
stages, this often results in blindness from
cataracts, nerve damage,
gangrene in the feet and legs leading to amputation, heart disease, and
kidney failure. Type 1 diabetes melitis (juvenile onset diabetes)
is due to decreased production of insulin by the pancreas. Type 2
diabetes melitis is due to increased resistance of cells in the body to
insulin. The gene or genes for diabetes are incompletely
penetrant.

a large organic
molecule that stores the genetic code for the synthesis of proteins.
DNA is composed of sugars, phosphates and bases arranged in a double helix
shaped molecular structure. Segments of DNA in chromosomes
correspond to specific genes.

a relatively common
genetically inherited abnormality of the X chromosome which results in
mental retardation. Since it is an X-linked
trait, males more often have it expressed in their phenotypes.
Most fragile-X males have large testes, big ears, narrow faces, and sensory
integration dysfunctions that result in learning disabilities. It is
likely to occur 1 in 1000 births. Approximately 1 in 700 females are
carriers of the gene for this trait.

the transference of genes
from one population to another, usually as a result of migration. The
loss or addition of individuals can easily change the gene
pool frequencies of both the recipient and donor populations--that is,
they can evolve.

units of inheritance
usually occurring at specific locations, or loci, on a chromosome.
Physically, a gene is a sequence of DNA bases that specify the order of
amino acids in an entire protein or, in some cases, a portion of a protein.
A gene may be made up of hundreds of thousands of DNA bases. Genes are
responsible for the hereditary traits in plants and animals.

evolution, or change
in gene pool frequencies, resulting from random
chance. Genetic drift occurs most rapidly in small populations.
In large populations, random deviations in allele frequencies in one
direction are more likely to be cancelled out by random changes in the
opposite direction.

the study of gene
structure and action and the patterns of inheritance of traits from parent
to offspring. Genetic mechanisms are the underlying foundation for
evolutionary change. Genetics is the branch of science that deals with
the inheritance of biological characteristics.

the full genetic
complement of an individual (or of a species). In humans, it is
estimated that each individual possesses approximately 2.9 billion base
units
in his or her DNA. See Human Genome Project.

a genetically inherited metabolic
disorder in which there is an imbalance of uric acid in the blood which causes a build up
of urates in joint areas. Symptoms include painfully inflamed joints, especially of
the feet and hands, that can become chronic and result in deformity. Usually, only
one joint is involved, most commonly the base of a big toe. Gout is a sex-controlled trait, usually being more severe in men.

an
X-linked genetically
inherited recessive
disease in which one or more of the normal blood clotting factors is not
produced. This results in prolonged bleeding from even minor cuts and
injuries. Swollen joints caused by internal bleeding is a common
problem for hemophiliacs. Hemophilia most often afflicts males.

the body's genetically inherited
system for recognizing and rejecting foreign tissues, such as transplanted organs.
The HLAsystem has the most genes of any other
known human multiple-allele series. There are
at least 30,000,000 possible HLA genotypes. HLA stands for "human
leukocyte antigen" .

chromosomes
that are paired during the production of of sex cells in
meiosis. Such
chromosomes are alike with regard to size and also position of the centromere. They also have the
same genes, but not necessarily the same
alleles, at the same locus
or location.

having the same allele
at the same locus on both members of a pair of homologous
chromosomes. Homozygous also refers to a genotype
consisting of two identical alleles of a gene for a particular trait.
An individual may be homozygous dominant (AA)
or homozygous recessive (aa). Individuals
who are homozygous for a trait are referred to as homozygotes. See heterozygous.

a multinational research effort designed to
identify and map the location of all human genes. The idea of a Human
Genome Project began at a 1984 international conference in Utah.
Research to decode the human genome began in earnest in 1986, funded by
the U.S. Department of Energy. The initial stage of discovering all
human DNA codons was completed
early in 2001 at a cost of 2.7 billion dollars. The next phase of
research will be to identify the
proteins for which these genes
code. This decoding of the human
proteome will be an even
more daunting task than the original Human Genome Project. See
genome.

a severe genetically
inherited fatal degenerative nerve disorder. The symptoms usually do
not appear until early middle age. There is a progressive loss of
muscle control that inevitably leads to paralysis and death. This
disease is also called Huntington's chorea.

the situation in which an
allele is expressed only if certain factors are present in the
environment. The triggering of genetically inherited diabetes
by obesity and possibly severe emotional stress is an example.

cell division in specialized tissues of ovaries and
testes which results in the production of
sperm or
ova. Meiosis involves two
divisions and results in four daughter cells, each containing only half
the original number of chromosomes--23 in the case of humans.

identical twins.
Twins that come from the same zygote
are essentially the same genetically. Differences between monozygotic twins later in life are
virtually always the result of environmental influences
rather than genetic inheritance. Fraternal twins may look similar but
are not genetically identical.

a genetically inherited progressive
disease of the central nervous system. MS occurs as a consequence of
one's own immune system attacking the insulating sheath that normally
protects neurons. Symptoms range from numbness and tingling to
paralysis. There is a loss of motor and cognitive functions. The gene(s) responsible for MS are
incompletely penetrant in that the onset of the disease
is apparently triggered by a virus
and possibly other environmental factors.
There is a correlation between the amount of
sunlight that children are exposed to and the likelihood that they will
develop MS later in life. People who spend much of their first 16 years
in tropical and subtropical regions of the world are much less likely to
develop this disease than those who live in far northern and far southern
regions of our planet. It is believed that the lack of abundant sun exposure early in
life is somehow responsible for the later onset of MS.

any of a group of inherited
progressive muscle disorders caused by a defect in one or more genes
that control muscle function. MD is characterized by a gradual, irreversible
wasting of skeletal muscle. It is a sex-linked trait
most often passed on to sons by their mothers. The most common form, Duchene MD,
begins to weaken the legs of boys by age 3 and inevitably gets worse with each passing
year. There is no cure for this disorder, and it usually results in death before the
age of 30.

an alteration of
genetic material such that a new variation is produced. For instance,
a trait that has only one allele (A) can mutate
to a new form (a). This is the only
mechanism of evolution that can produce new
alleles
of a gene.

Charles Darwin's incorrect theory
about the inheritance of traits. He proposed that hereditary particles
in the body are affected by the things an individual does during his or
her lifetime. These modified particles were thought to migrate via blood
to the reproductive cells and subsequently could be inherited by the next
generation. This was a variation of Lamarck's incorrect idea of the
"inheritance of acquired characteristics."

an inherited trait that is
determined by genes at two or more loci. Simple Mendelian rules of
dominance do not apply to the complex interaction of these genes. As a result,
phenotypes may appear as apparent blends or
intermediate expressions. Human skin and hair
color are polygenic traits. Many polygenic traits are also influenced
by environmental factors.

Gregor Mendel's second principle of
genetic inheritance. It states that different pairs of genes
are passed to offspring independently so that new combinations of genes, present in
neither parent, are possible. In other words, the distribution of one
pair of alleles does not influence the distribution of another pair.
The genes controlling different traits are inherited independently of one
another.

Gregor Mendel's first principle of
genetic inheritance. It states that, for any particular trait, the pair of
genes of each parent separate (during the formation of sex cells) and
only one gene from each parent passes on to an offspring. In other
words, genes occur in pairs (because
chromosomes occur in pairs).
During gamete production, the members of each gene pair separate, so that
each gamete contains one member of each pair. During fertilization,
the full number of chromosomes is restored, and members of gene pairs are
reunited.

any of a large number of complex
organic molecules that are composed of one or more chains of amino
acids. Proteins can serve a wide variety of functions through
their ability to bind to other molecules. Proteins may be enzymes, hormones, antibodies, structural components, or
gas-transporting molecules.

a simple graphical method of
showing all of the potential combinations of offspring genotypes
that can occur and their probability given the parent genotypes. See
example below. Punnett squares are commonly used
by genetics counselors to predict the odds of a couple passing on
particular inherited traits.

a gene that can
initiate or block the functions of other genes. Regulator genes control the timing
of production of a variety of chemicals in humans and other organisms.
Shortly after conception, regulator genes work as master switches
orchestrating the timely development of our body parts. They are also
responsible for changes that occur in our bodies as we grow older.
Regulator genes are also called homeotic
genes.

a genetically inherited
recessive condition in which red blood cells are distorted
resulting in severe anemia and related symptoms that are often fatal in childhood.
Sickle-cell trait is the result of a pleiotropic gene.
Sickle-cell trait is also known as sickle-cell anemia.

defective alleles
that have segments which are doubled in their transmission from generation to
generation. In the case of genetically inherited diseases, the result is
increasingly severe symptoms each generation. The myotonic form of muscular dystrophy is an example. Stuttering
alleles are also known as unstable alleles.

a genetically
inherited condition caused by the inability to produce the enzyme hexosaminidase A. This results in progressively increased fluid
pressure on the brain and the subsequent degeneration of the brain and nervous system
beginning about 6 months of age and inevitably resulting in death usually by
age 2-3. The gene responsible for Tay-Sachs Disease is
recessive. It has been most common among the descendents of Eastern
European Jews (Ashkenazi Jews).

Gregor Mendel's idea that the
characteristics of parents are passed on to descendants unchanged as units. In other
words, the hereditary material of any organism is made up of discrete units (now called
genes).

a category of extremely small
microscopic parasites of plants, animals, and bacteria. Viruses are not cells but rather
RNA or DNA molecules surrounded by a protein coating. Since
viruses cannot reproduce without a host cell, they are not strictly speaking living
organisms.

a
"fertilized" ovum. More precisely, this
is a cell that is formed when a sperm and an ovum
combine their chromosomes at conception. A
zygote contains the full complement of chromosomes (in humans 46) and has
the potential of developing into an entire organism.